It is known that the discharge lamps, such as cold cathode fluorescent lamp (CCFL), are often being used as the backlight of liquid crystal displays. The cold cathode fluorescent lamps are generally driven by alternating current (AC) with frequency matching the specification of the CCFLs.
To ignite or first turn on a CCFL, the circuit for driving the CCFL must provide a momentary strike or startup voltage that is typically more than 1000 V and is usually referred as the discharge voltage, starting voltage or striking voltage. After switching to normal running, the operating voltage of the CCFL is generally in a range from 300V to 700V, that is about one half or one third the starting voltage, depending on the type of the CCFL. When current flows though the tube, the impedance of the tube decreases and the voltage across the tube drops rapidly. When current flows to a particular level, the decline of tube voltage stops and the CCFL shows an almost steady voltage, the voltage at this time is called tube voltage, operating voltage or running voltage. It is necessary to keep the current flowing after startup. The tube current is directly proportional to the CCFL brightness, increasing the CCFL current increases the brightness, however too much current may damage the electrode and lead to a shorter lifetime. Generally, 3 mA to 7 mA is commonly used for each CCFL. CCFLs are generally driven by alternating current (AC), the AC frequency typically ranges from 30 kHz to 100 kHz.
The CCFLs are typically driven by a DC to AC inverter, which generally provides a wide range of DC input voltage and transforms the voltage into an AC high voltage and high frequency output to run the lamp. However, CCFLs exhibit a negative impedance characteristic which makes the series resistance measured in the CCFL tube to decrease rather than to increase as desired when the current flowing therein is increased. Therefore, The inverter used for driving CCFLs must be able to provide an adjustable AC power and a feedback circuit for ensuring the stability of the driving circuit of CCFLs while allowing the loading of CCFLs to be adjusted.
Conventionally, the brightness of a CCFL is controlled by a power driving means that is integrated in an application specific integrated circuit (ASIC). However, since CCFLs of different tube size will require to be driven by different driving frequency, there should be as many ASICs specifically designed to meet the requirement of those CCFLs. That is, there is a specifically designed ASIC for a specific CCFL, that is not economically sound with respect to modern industrial standard. Therefore, it is in need of a micro control unit (MCU) which can be programmed and adapted for controlling CCFLs of different specifications. It is not only intended to control various CCFLs by using a same MCU, but also to save the manufacturing cost of CCFLs accordingly.